Spark plug



octr3, v w.,w. HIGGINS 2,359,421

. SPARK PLUG Filed Oct. 10, 19:10

' F1111. Fla-3.

: INVENTOR.

5/ .1 ,i A J 11: W 7 5 7 i 7' v V Waldo WHzlggins FIG. 2.

Patented Oct. 3, 1944 2.559.421 SPABK PLUG wuss w. Higgins. Beams. amt.

to A. O. Smith Corporation, am fiw e fl my Milwaukee, ,Wis., a j

Applicationoctober 10,1940. Serial to. 366,614

This invention relates to a spark plug.

Heretofore spark plugs have beenconstructed V with theparts mechanically assembled so that different parts are replaceable. 'In such spark plugs thecore and shell are scaled by a gasket,

or by a special cement, or by both a gasket and cement. However, tests show that there is always some leakage of gas through the seal making for loss in engine efllciency. r

,. Furthermore; in the usual automobile engine spark plug porcelain is.used as a dielectric. It

' 'is subject to checking-and cracking in time, re-

: quiring frequent replacement. -In aviation spark plugs mica .is' usually employed as a dielectric.

= These'plugs are very expensive to construct, and

inspite of the care taken in their manufacturing theugllal life f j t1 sixty hours- I I In both the automobile and the aviation type of plug the'customary practice is to replace "theentire plug when a defect shows up, rather-than to replace only a part. This'fact, coupled'with' modern methods for cleaning plugs, makes it entirely unnecessary to have mechanically assembled plugs.

The object of the present invention is to provide a spark plug which is unitary and integral in construction, thereby eliminating the mechanical seal between the core and shell.

Another object is to provide a spark plug in which the shell and central electrode are integrally joined by a dielectric material fused therebetween.

Another obiect is to provide a spark plug in I which a fused dielectric material bonds the central electrode in place in the shell and is the sole means for holding the electrode against the working pressures of the engine.

Another object is to provide a central electrode for a spark plug, with a fused coating of dielectric material bonded thereto.

Another object is to provide a spark plug which is less costly to construct and which will satisfactorily perform in service.

Claims. (cr es-ice n I Fig. 2 is a transverse section taken on line 2-2 of Fig. 1; I

' Fig. 3 is a view similar 1 of the parts c after assembly andprior to fusion; and

Fig. 4 is a view similar to Fig. 2, of the parts after assembly and prior to fusion.

The spark plug illustrated in the drawing and embodying the invention -is of the aviation type.

.It has ametal shell I of generally cylindrical shape and having a central circumferential shoulder 2 for seating against sealing means on the engine block. External threads 3 are provided on the lower end'of the shell for securing the plug in the engine block. v

- The inside of the shell I isiof varying diameter, and is preferably divided into four sections longitudinally of the shell. The two central sec- "tions are of smallest internal diameter, the lower central section beingof'substantially' uniform diameter, and the'upper central section having a slight outward taper upwardly'fro'm the center.

' This taper should preferably be such as to make the largest diameter of thissection about fortythousandths of an inch larger than the smallest diameter. The uppermost section is relatively short and has its walls tapered at a greater angle than the next lower section. The bottom section has its walls tapered outwardly toward the bottom to provide a space for receiving the points 4 which are welded to the shell.

The inner surface of the shell I has a continuous coating 5 of vitreous enamel fused thereto as a dielectric.

The central electrode comprises a rod 6 of substantial diameter having a head i at the top for connection with the terminal of an ignition wire, not shown, and an alloy tip 8 welded to its lower end for cooperation with the points 4 in providing a spark gap. The rod 6 has a continuous coating 9 of vitreous enamel .fused thereto as a dielectric.

In the manufacture of the spark plug, after the central electrode and outer shell are constructed as described, they are placed in a suitable Jig for positioning them in final relation. Then powdered frit is packed into the space between the two, from the top, and the annular groove l0 provided between the shell and central electrode at the top is filled with additional frit II, as shown in Fig. 3.

The plug is then placed in a furnace where it is heated to a suitable temperature for fusing the enamel frit, usually from 1500' F. to 1"100 F. The

finished plug has the enamel coatings 5 and 9 fused together by the frit ll forming an integral dielectric which joins the rod 8 and shell I.

In constructing the spark plug it has been found that'the former relatively small diameter central electrode is unsuitable for enameling. The preferable diameter is from one-quarter to three-eighths of an inch, since with this curvature of surface, the enamel will flow properly to provide a continuous coating free from shrink strains and defects.

The fusing of the two enamel coatings 5 and 9 together is not solely dependent upon the added frit, and it is desirable to have the rod 8 and its coating 9 fit closely within the opening in the shell. A tolerance of forty thousandths of an inch space can be allowed between the two coatings in obtaining a satisfactory interfusing of the same. This space, however, may be varied to some extent by the type of enamel employed.

For the purpose of forming coatings 5 and 9 it is preferable to employ an enamel having good bonding properties since defect of bond will result in early breakdown of the dielectric. The coatings should be of substantial thickness, preferably from twenty-five to forty thousandths of an inch, and may be formed by the successive fusing of several layers of enamel.

The head I and tip 8 on the central electrode may be of any suitable construction depending upon the use contemplated for the plug. The points 4 may also be of different construction.

The invention provides a plug of few parts and of relatively inexpensive construction. Various embodiments of the invention may be employed within the scope of the accompanying claims.

The invention is claimed as follows:

1. A spark plug comprising a shell having means for securing the plug to an engine, a central electrode having a body of substantially uniform diameter disposed axially of the shell,-the inner surface of the shell providing with the electrode a tapering recess in the upper portion therebetween and a closely fitting central portion, and

a vitreous dielectric material filling the recess and the space in the central portion and fused and bonded to the adjacent surfaces of the shell and electrode.

5 2. The method of providing a dielectric between the central electrode of a spark plug and the shell, comprising applying a vitreous enamel coating on the or posing surfaces of said core and shell, assembling the core and shell concentrically with a.

10 small ace therebetween providing a tapered annular recess at the top, filling the recess with enamel frit, and fusing the frit and enamel coatings into an integral dielectric bonded to the surfaces of the core and shell and providing a gas tight pressure resisting joint therebetween.

3. In the manufacture of a spark plug, the method of making the central electrode therefor, comprising providing a metal rod of not less than about one quarter of an inch in diameter and of substantially uniform diameter throughout, providing a head thereon for receiving an electrical connection, securing an alloy tip at the end opposite the head, and applying a continuous coating of substantial thickness of vitreous enamel fused and bonded to the outer surface of the rod.

4. A spark plug comprising an outer tubular shell and a central electrode providing a tapered space therebetween, said members being secured together and insulated from one another by a vitreous enamel packed and fused in said tapered space.

5. A spark plug comprising an outer tubular shell and a central electrode providing a tapered space therebetween, said core being of not less than about one-quarter inch in diameter and having a vitreous enamel coating fired and bondedto its outer surface, a similar vitreous enamel coating fired and bonded to the inner surface of the outer shell, and a powdered enamel o frit packed in said tapered space between the members and fused and bonded to the respective enamel coatings.

WALDO W. HIGGINS. 

